Move the call_rcu_thread explicit create and shutdown to isc_loop

Move the call_rcu_thread explicit create and shutdown to isc_loop

When isc__thread_initialize() is called from a library constructor, it
could be called before we fork the main process.  This happens with
named, and then we have the call_rcu_thread attached to the pre-fork
process and not the post-fork process, which means that the initial
process will never shutdown, because there's noone to tell it so.

Move the isc__thread_initialize() and isc__thread_shutdown() to the
isc_loop unit where we call it before creating the extra thread and
after joining all the extra threads respectively.